1. Field of the Invention
The present invention relates to a multi-system television receiver having a subcarrier generation circuit which is operable on a number of different systems.
2. Description of the Prior Art
In the world, there are a number of different types of television broadcasting systems, such as, PAL system, SECAM system, NTSC system and others. In some places in the world, television broadcasting with two or more systems is available. Also, video tapes recorded under different systems are available.
To enable the reproduction of color television signals of different systems, a color television receiver which can receive and reproduce color television signals of different systems has been developed. Such a television receiver is referred to as a multi-system television receiver.
The signal format for the NTSC system and that for the PAL system are very similar to each other. Therefore, in the prior art multi-system television receiver, the color signal processing circuit, particularly the subcarrier wave oscillator, an APC phase detector and a killer phase detector are used in common for both systems. Furthermore, the multi-system television receiver has a detecting means for detecting the type of television system being received so as to switch various circuits which are necessary to process the television signal to a viewable image on a CRT.
For example, according to the PAL system, a subcarrier wave having a frequency of 4.43 MHz is used in most places, but in some places, a 3.58 MHz subcarrier wave is used. As to the NTSC system a, 3.58 MHz subcarrier wave is used normally. Furthermore, some VRT tapes recorded under NTSC system use 3.58 MHz and others use 4.43 MHz subcarrier wave.
Accordingly, the prior art multi-system television receiver which can receive television signals in PAL system and NTSC system, or which can reproduce VTR tapes recorded under various systems, employs two oscillators for generating two subcarrier waves having frequencies of 4.43 MHz and 3.58 MHz. For this purpose, a suitable switching means is provided for selecting one oscillator for generating the required subcarrier waves.
Therefore, the prior art multi-system television receiver requires two oscillators, resulting in high manufacturing cost. Also, a suitable switching means is necessary to select one oscillator for the particular system signal being received.
To remove the above described disadvantage, a multi-system television receiver having an improved subcarrier generation circuit has been proposed by the same inventor as the present invention, and is disclosed in Japanese Patent Application laid-open publication No. 58-152070, which is assigned to the same assignee as the present application. According to this publication, only one oscillator is provided which may generate subcarrier waves at different frequencies, and automatically selects and produces a subcarrier wave having an appropriate frequency for the received television signal. A detail of the subcarrier generation circuit disclosed in this publication will be described below in connection with FIG. 1.
In FIG. 1, reference number 1 designates a burst gate circuit which, in accordance with the burst gate pulse in the horizontal scanning period, separates and generates a burst gate signal from the chrominance signal. Reference number 2 is a phase detector for the automatic phase controller (APC), 3 is a low pass filter, and 4 is a voltage-controlled oscillator (VCO). These circuits 2, 3 and 4 define an automatic phase controller (APC), and thus VCO 4 produces a subcarrier signal.
VCO 4 is provided with two crystal vibrators 5a and 5b for generating a signal at a frequency 3.58 MHz or 4.43 MHz. A switching circuit 6 is provided for selectively connecting one crystal vibrator to VCO 4 in response to the frequency of the burst signal.
Further provided is a phase detector 7 for a killer. When phase detector 7 stops producing the normal signal, flip-flop 8 reverses its condition upon receipt of a driving pulse having a pulse width of about 200 milliseconds. The output of flip-flop 8 is connected to switching circuit 6 for selecting one vibrator.
The operation of the subcarrier generation circuit of FIG. 1 is as follows. When a television signal of the NTSC system is received, a burst signal having a frequency 3.58 MHz is separated and produced from burst gate circuit 1. The burst signal is applied to both phase detectors 2 and 7. In this case, switching circuit 6 is so actuated as to connect crystal vibrator 5a with VCO 4 in accordance with the output signal from flip-flop 8, and at the same time, the APC loop operates in a synchronized relationship with the burst signal at a frequency 3.58 MHz. Thus, VCO 4 produces a subcarrier wave having a frequency 3.58 MHz. In this manner, once the APC loop is synchronized, phase detector 7 produces a phase detected signal which suspends the further operation of flip-flop 8. Thus, VCO 4 produces the normal 3.58 MHz subcarrier wave in a stable condition. Then, if for some reason, the control of flip-flop 8 delays, switching circuit 6 may be so operated as to connect crystal vibrator 5b (4.43 MHz) to VCO 4. In such a case, the signal in the APC loop becomes mis-synchronized, thereby no output signal is produced from phase detector 7. Thus, flip-flop 8 changes its condition upon receipt of a next drive pulse, thereby switching the switching circuit 6 to connect crystal vibrator 5a (3.58 MHz) to VCO 4. Accordingly, the APC loop is synchronized at a frequency of 3.58 MHz, so that VCO 4 produces a normal subcarrier wave at frequency 3.58 MHz.
Next, when a television signal of the PAL system is received upon changing the channel, a burst signal having a frequency 4.43 MHz is separated and produced from burst gate circuit 1. Accordingly, by the output signal from flip-flop 8, switching circuit 6 is actuated to connect crystal vibrator 5b to VCO 4. Thus, APC loop operates in a synchronized relationship with the burst signal at a frequency 4.43 MHz. Thus, VCO 4 produces a subcarrier wave having a frequency 4.43 MHz. In this manner, once the APC loop is synchronized at frequency 4.43 MHz, phase detector 7 produces a phase detected signal which suspends the further operation of flip-flop 8. Thus, VCO 4 produces the normal 4.43 MHz subcarrier wave in a stable condition.
According to the prior art subcarrier generation circuit described above, there are a number of disadvantages as described below.
The first disadvantage is about an error operation which may take place when a television signal of the SECAM system is received.
According to the SECAM system, chrominance signal contains a subcarrier wave at a frequency 4.25 MHz or 4.406 MHz, which is very close to the frequency 4.43 MHz of the burst signal. Accordingly, if the signal component at the trailing edge of the horizontal sync signal where the burst signal of PAL system or NTSC system is located should come in while the television signal of the SECAM system is being received, the APC circuit makes a retraction operation, resulting in an error operation as if the burst signal at 4.43 MHz is received.
The second disadvantages is about an error operation which may take place upon change of the channel from a channel in one broadcasting system, such as a PAL system, to a channel in another broadcasting system, such as a SECAM system.
More specifically, according to the prior art subcarrier generation circuit, the low pass filter defining the APC loop has a relatively long time constant. Therefore, when the channel change between two different broadcasting systems is effected, it takes a relatively long time to detect the broadcasting system of the newly selected channel and to switch the switching circuit 6 so as to generate a proper subcarrier wave from VCO 4. During this period of time, the image on the screen is often distorted.
The last disadvantage is about an error operation caused by a noise signal produced from flip-flop circuit 8 or a system detection circuit for detecting the type of broadcasting system now receiving.
Specifically, according to the prior art multi-system television receiver, the system detection circuit detects and produces a signal representing the type of broadcasting system being received. However, the system detection circuit makes an error detection by the noise signal. The noise signal is, for example, contained in the received broadcasting signal, or produced upon reproduction of a tape recorded under poor conditions, or produced when the video search signal is added.